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A flight number, when combined with the name of the airline and the date, identifies a particular flight. This callsign should not be confused with the tail number of the aircraft, although both can be used as a call-sign as used in general aviation. A particular aircraft may fly several different flights in one day, and different aircraft may be used for the same flight number on successive days.
A number of conventions have been developed for defining flight numbers, although these vary widely from airline to airline. Eastbound and northbound flights are traditionally assigned even numbers, while westbound and southbound flights have odd numbers. Other airlines will use an odd number for an outbound flight and use the next even number for the reverse inbound flight. For destinations served by multiple flights per day, numbers tend to increase during the day. Hence, a flight from point A to point B might be flight 101 and the return flight from B to A would be 102, while the next pair of flights on the same route would usually be assigned codes 103 and 104.
Flight numbers of less than three digits are often assigned to long-haul or otherwise premium flights. Flight number 1 is often used for an airline's "flagship" service. For example, British Airways flight 1 was the early morning supersonic Concorde service from London to New York City; Air New Zealand flight 1 is the daily service from London to Auckland via Los Angeles; American Airlines Flight 1 is the daily flight from New York to Los Angeles; and El Al flight 1 is the daily overnight service from Tel Aviv to New York City. Four-digit numbers in the range 1000 to 4999 typically represent regional affiliate flights, while numbers larger than 5000 are generally codeshare numbers for flights operated by different airlines or even railways.
Likewise, flight numbers larger than 9000 are usually referred to as ferry flights, that carry no passengers and are only to move an aircraft from point A to point B, where it is supposed to start a new commercial flight. Flight numbers starting with 8 are often used for charter flights, but it always depends on the commercial carrier's choice.
Flight numbers are often taken out of use after a crash or a serious incident. For example, following the crash of Alaska Airlines Flight 261, the airline changed the flight number for subsequent flights following the same route to 229. Also, American Airlines Flight 77, which regularly flew from Dulles International Airport in Washington, DC, to Los Angeles International Airport, was changed to Flight 149 after the September 11, 2001 attacks. On the other hand, other considerations may lead an airline not to change a flight number; for instance, the aforementioned "flagship" American Airlines Flight 1 retains its designation despite a major accident in 1962. The most recent flight number change was from Indonesia AirAsia Flight 8501 to Indonesia AirAsia Flight 678, according to AirAsia's online booking system.
Note that, although 'flight number' is the term used colloquially, the official term as defined in the Standard Schedules Information Manual (SSIM) published annually by the International Air Transport Association (IATA) Schedules Information Standards Committee (SISC), is flight code. Officially the term 'flight number' refers to the numeric part (up to four digits) of a flight code. For example, in the flight codes BA2490 and BA2491A, "2490" and "2491" are flight numbers. Even within the airline and airport industry, it is common to use the colloquial term rather than the official term.
Flight numbers are also sometimes used for spacecraft, though a flight number for an expendable rocket (say, Ariane 5 Flight 501) might more reasonably be called the serial number of the vehicle used, since an expendable rocket can only be launched once. Space Shuttle missions used numbers with the STS prefix, for example, STS-93.